Many components used in fiber optic communication systems impart a partial polarization onto the optical signal propagating along the fiber and through the components. This characteristic is often defined as “polarization dependent loss” (PDL) and is considered an undesirable property since PDL can lead to amplitude modulation distortion in analog communication systems and increased bit error rate in digital communication systems. A further problem is that PDL is often a dynamic quantity, as a result of separate losses associated with concatenated components, as well as the fact that the optical fiber connecting components may randomly transform the polarization of the optical signal. This random transformation is affected by external conditions such as temperature and the stress on the fiber, which may vary with time.
In order to mitigate the deleterious effects of PDL, it is necessary to be able to accurately measure, simulate, and (if necessary) compensate for PDL. For all of these applications, it is desirable to have a device that can produce a specified amount of PDL. In order for a PDL device to be useful, it must have two properties: (1) the value of the polarization dependent loss itself must be “tunable”; and (2) the principle states of polarization (PSP) (i.e., the polarization states of maximum and minimum insertion loss) must also be tunable. In order to achieve these two properties, most tunable PDL schemes utilize a PDL element with variable loss (but fixed polarization states) in conjunction with a polarization controller disposed at the input of the PDL. The polarization controller allows for the PSP to be varied at the input of the PDL element, thus meeting the first requirement.
Although this arrangement addresses the needs, such arrangements may require two or more such PDL elements to be concatenated to provide truly “tunable” PDL. Moreover, polarization control devices are relatively expensive and exhibit high insertion loss. Further, tunable PDL elements that exist in the prior art, such as that disclosed in U.S. Pat. No. 6,347,164 require the use of bulk optic components (e.g., lenses) that are large and unstable.